The properties of polymers can be modified by crosslinking of the polymer chains. One method for crosslinking which is commonly used is known as "silane crosslinking", i.e., crosslinking using olefinically unsaturated alkoxysilanes. This method involves at least two stages, which may be done together. In the first stage, (a) silane is grafted onto polymer chains in the presence of a free radical generator and in a second stage, (b) crosslinking of the polymer chains occurs through the formation of Si--O--Si bonds as bridges. The grafting step is normally effected while the polymer is in a molten state, most commonly in an extruder, while the crosslinking step is normally carried out after the polymer has been formed into its desired shape, such as a pipe or wire insulation, while the polymer is in a solid state. To effect crosslinking, the intermediate formed by grafting is subjected to moisture, for example, hot water or steam, in the presence of a hydrolysis/condensation catalyst. The two processes most often employed are (1) a two-step process as taught in U.S. Pat. No. 3,646,155 and (2) a one-step process as taught in U.S. Pat. No. 4,117,195.
One important commercial application of silane crosslinking is to crosslink ethylene polymers used as insulation and sheathing compositions for electrically or optically conductive wire and cable. In this application, it is often necessary for the crosslinked insulation to survive extended periods of exposure to heat, light, and air, during which the polymer may undergo degradation such as by embrittlement or discoloration. To prevent or reduce such degradation, it is often desirable to include in the crosslinked polymer one or more stabilizer compounds.
Many such stabilizer compounds reduce oxidative degradation of polyolefins by intercepting free radicals in the polymer and converting them to unreactive forms. Unfortunately, the silane grafting stage described above also involves free radical processes, and therefore stabilizers may interfere with the silane grafting step. Therefore a problem associated with silane crosslinking employing such stabilizer compounds is the difficulty of accurately combining some or all of the silane, free radical generator and hydrolysis/condensation catalyst (some or all of which are liquids) and stabilizers with the base polymer which is a solid to minimize this interference.